ABSTRACT Mitochondria uptake calcium via a calcium activated channel called the uniporter. Calcium uptake allows the organelle's metabolic state to be matched to rapidly changing energy requirements, and, in turn, tune key processes such as neurotransmission and muscle contraction. The uniporter is calcium-activated calcium channel regulated by the calcium binding heterodimer MICU1/MICU2. Mutations in MICU1 have recently been identified as a cause of a new form of a myopathy characterized by fatigue and exercise intolerance without the classical features of mitochondrial myopathy. The precise mechanisms by which MICU1 and MICU2 sense calcium to regulate the uniporter, and how lesions in this heterodimer lead to this highly unusual myopathy are not known. Through this dual PI grant we propose to: (1) Characterize the physicochemical properties of MICU1 and MICU2. The wild type proteins and mutants expressed in E. coli will be characterized with respect to the oligomeric state, structural stability, Ca2+ and Mg2+ binding affinities, and pH sensitivity. (2) Determine the structural basis for the regulation of the uniporter by MICU1 and MICU2. High resolution X-ray structures of MICU2 alone and of the MICU1/2 heterodimer in the apo and Ca2+-bound forms will be determined. Electron cryo-microscopy will be used to determine the structure of a native-like oligomer of MICU1/2 complex. (3) Investigate mitochondrial calcium dynamics in cellular systems. Using genome-editing technology we have engineered a powerful in vivo system of knockout cell lines for studying the effects of engineered and naturally occurring human mutations in MICU1 and MICU2 on the mitochondrial calcium transport kinetics and energetics, and (4) Understand the metabolic and bioenergetic basis of human MICU1 myopathy by investigating the Micu1-/- mouse as a model. We will characterize the muscle histology, mitochondrial bioenergetics, exercise performance and metabolomics, and single fiber contractility to test the hypothesis that loss of MICU1 leads to a myopathy by causing disturbances in energy metabolism. Our aims ? spanning the molecular to animal physiology -- will yield a holistic and mechanistic understanding of the regulation of mitochondrial calcium uptake by MICU1 and MICU2 and its contribution to a newly described human myopathy.